Empty values and commented lines will result in the default value, if any,
being selected. If a setting is set, but not used by a given
filtertype, it may generate an error.

This filtertype will iterate over the actionable list and match
indices matching a given pattern. They will remain in, or be removed from
the actionable list based on the value of exclude.

Filter chaining

It is important to note that while filters can be chained, each is linked by an
implied logical AND operation. If you want to match from one of several
different patterns, as with a logical OR operation, you can do so with the
pattern filtertype using regex as the kind.

This example shows how to select multiple indices based on them beginning with
either alpha-, bravo-, or charlie-:

Explaining all of the different ways in which regular expressions can be used
is outside the scope of this document, but hopefully this gives you some idea
of how a regular expression pattern can be used when a logical OR is desired.

A word about regular expression matching with timestrings

Timestrings are parsed from strftime patterns, like %Y.%m.%d, into regular
expressions. For example, %Y is 4 digits, so the regular expression for that
looks like \d{4}, and %m is 2 digits, so the regular expression is \d{2}.

What this means is that a simple timestring to match year and month, %Y.%m
will result in a regular expression like this: ^.*\d{4}\.\d{2}.*$. This
pattern will match any 4 digits, followed by a period ., followed by 2 digits,
occurring anywhere in the index name. This means it will match monthly
indices, like index-2016.12, as well as daily indices, like
index-2017.04.01, which may not be the intended behavior.

To compensate for this, when selecting indices matching a subset of another
pattern, use a second filter with exclude set to True